One specific area in the Federal Republic of Germany experienced a particularly high contamination by fallout from the Chernobyl reactor accident in 1986. The individuals living there received an additional radiation exposure from the fallout of about 0.8 mSv in 1986, which had decreased to about 0.2 mSv in 1989. Blood samples for chromosome analyses were collected from 29 different individuals during the period 1987-1991. About 44,000 cells were scored. The individuals were subdivided according to sampling time. In 1987/88, the mean frequency of dicentrics and centric rings (dic + Rc) was 2.45 +/- 0.42 per 1000 cells and that for excess acentrics (ace) was 8.20 +/- 0.90 per 1000 cells. In the following 2 years the mean yields per 1000 cells for dic + Rc were 1.60 +/- 0.40 and 1.79 +/- 0.42, respectively, and that for ace 8.18 +/- 0.90 and 6.92 +/- 0.83, respectively. The mean yields for chromatid breaks (cbr) per 1000 cells in the same periods were 4.96 +/- 0.60, 6.79 +/- 0.82, and 5.14 +/- 0.71, respectively. In addition, individuals were investigated as controls in two different areas where the exposure doses from the Chernobyl fallout were lower by one order of magnitude. In one control area, the mean frequency of structural chromosome aberrations was found to be 2.58 +/- 0.36 dic + Rc, 9.62 +/- 0.69 ace, and 6.54 +/- 0.57 cbr per 1000 cells (20 individuals, approximately 20,000 cells). In the other control area, the mean yields were 1.91 +/- 0.42, 8.63 +/- 0.89, and 5.90 +/- 0.73, respectively, per 1000 cells (11 individuals, approximately 11,000 cells). These yields show no significant trend, either with sampling time or with contamination level for any of the three aberration types. Five individuals from the highly contaminated area who were investigated in 1987, were reinvestigated in 1991. On the basis of about 10,000 cells, the mean yield of dic + Rc per 1000 cells decreased significantly from 2.69 +/- 0.52 to 1.40 +/- 0.37 (p < 0.05). In nine individuals from the contaminated area, the content of cesium nuclides was measured using a whole body counter. No relation between the physically calculated radiation doses to blood and the frequency of dicentrics determined in 1987/88 was found (r = 0.41).